Wireless networking devices have a finite communication range which is limited by the output power of the transmitting device, the signal attenuation in the wireless channel and the sensitivity of the receiving device. The limitations can be regulatory restrictions (maximum allowed output power), technical constraints (size of transmitting and receiving antennas, electrical power consumed by transmitting device or receiving device), or the physical environment (e.g., obstructing walls).
Furthermore, wireless links are not necessarily symmetric. A link from node 1 to a node 2 does not imply a link in the reverse direction. The reasons for such asymmetry can be technical (such as differences (tolerances) in receiver sensitivity between the two nodes or deliberate transmission power reduction in order to conserve battery power), due to the environment in which the nodes operate, or due to the device construction itself (in case of simple transmit-only devices).
A wireless network suitable for hearing instruments may be an inductive link connecting a right-ear hearing instrument and a left-ear hearing instrument with each other and with accessory devices (typically body-worn) comprising a microphone and/or acting as an interface to external devices, such as a phone or mobile phone.
Another type of wireless network for hearing instruments uses an electromagnetic (i.e., far-field) link for connecting remote accessory devices, such as a wireless microphone, to an ear-level receiver device.
Typically, such remote wireless microphones are used by teachers teaching hearing impaired persons in a classroom (wherein the audio signals captured by the wireless microphone of the teacher are transmitted to a plurality of receiver units worn by the hearing impaired persons listening to the teacher) or in cases where several persons are speaking to a hearing impaired person (for example, in a professional meeting, wherein each speaker is provided with a wireless microphone and with the receiver units of the hearing impaired person receiving audio signals from all wireless microphones). Another example is audio tour guiding, wherein the guide uses a wireless microphone.
Another typical application of wireless audio systems is the case in which the transmission unit is designed as an assistive listening device. In this case, the transmission unit may include a wireless microphone for capturing ambient sound, in particular from a speaker close to the user, and/or a gateway to an external audio device, such as a mobile phone; here the transmission unit usually only serves to supply wireless audio signals to the receiver unit(s) worn by the user.
U.S. Patent Application Publication 2005/0195996 A1 and corresponding U.S. Pat. No. 8,019,386 B2 relate to a hearing assistance system comprising a plurality of wireless microphones worn by different speakers and a receiver unit worn at a loop around a listener's neck, with the sound being generated by a headphone connected to the receiver unit, wherein the audio signals are transmitted from the microphones to the receiver unit by using a spread spectrum digital signals. The receiver unit controls the transmission of data, and it also controls the pre-amplification gain level applied in each transmission unit by sending respective control signals via the wireless link.
International Patent Application Publication WO 2008/098590 A1 relates to a hearing assistance system comprising a transmission unit having at least two spaced apart microphones, wherein a separate audio signal channel is dedicated to each microphone, and wherein at least one of the two receiver units worn by the user at the two ears is able to receive both channels and to perform audio signal processing at ear level, such as acoustic beam forming, by taking into account both channels.
One option to implement a wireless network for hearing instruments is to use a time division multiple access (TDMA) scheme with central or distributed network management, wherein one of the devices acts as a master and determines the time slots during which each network device may transmit data and communicates this transmission schedule to the participating devices. A device entering the network must first register with the network master and can subsequently request reservation of time slots to transmit its data to recipient devices, i.e., a device is allowed to transmit only once it has synchronized with the network. In case that the network uses a frequency hopping scheme, the master transmits the necessary information to the synchronized devices.
An example of a wireless network for hearing devices is described in U.S. Patent Application Publication 2010/0166209 A1 and corresponding U.S. Pat. No. 8,150,057 B2, wherein audio signals are transmitted from a plurality of wireless microphones worn by various speakers via the wireless network to a receiver unit worn by a listener. Each of the transmission devices has certain dedicated slots in the TDMA scheme for transmitting the audio signals captured by the microphone to the receiver unit.
Another example of a wireless network suitable for audio signal transmission is the BLUETOOTH® standard.
U.S. Patent Application Publication 2010/0158292 A1 and corresponding U.S. Pat. No. 8,194,902 B2 relate to a wireless network comprising a binaural hearing aid and other components, like a mobile phone, wherein a TDMA structure is used in a frequency hopping scheme, and wherein, during acquisition/synchronization, the frequency hopping scheme has a reduced number of frequency channels and the new device activates its receiver at a rate different from the frame rate of the network until synchronization data is received from the master device, whereupon the frame timing of the new device is synchronized to the frame timing of the network.
In the above-discussed systems, only members of the network, i.e., nodes which are synchronized to the network clock of the master device, are allowed to send messages to other nodes in the network. Thus, each device that wants to send a message to a network member has to synchronize to the network first, and therefore, has to receive messages from at least one of the network members, namely the master device. This imposes certain restrictions on such a device and also requires a certain time before a message can be sent, namely the time needed for synchronizing to the network.